Ab Initio Quantum Transport Calculations Research Articles

Giant intrinsic magnetoresistance in spin-filtered tunnel junctions with ferrimagnetic electrode

In recent years, magnetic tunnel junctions (MTJs) have attracted strong research interest due to their potential use in nonvolatile memory technologies, such as magnetoresistive random access memory and magnetic logic applications. Half-metallic materials have been proposed as ideal electrode materials for MTJs to achieve large tunnel magnetoresistance (TMR) effects. Here, we design and investigate a spin-filter MTJ (sf-MTJ) consisting of a ferrimagnetic inverse Heusler alloy, ${\mathrm{Mn}}_{2}\mathrm{CoSi}$ as the electrode and CaS as the insulating tunnel barrier using ab initio quantum transport calculations. Our results demonstrate a high zero-bias voltage TMR ratio that initially oscillated before decreasing as the bias voltage increased. Despite the oscillatory TMR under bias voltage, the spin injection remains high and stable, highlighting the potential of sf-MTJs formed by ${\mathrm{Mn}}_{2}\mathrm{CoSi}$ electrodes for practical applications.

Ab Initio Study of Magnetic Tunnel Junctions Based on Half-Metallic and Spin-Gapless Semiconducting Heusler Compounds: Reconfigurable Diode and Inverse Tunnel-Magnetoresistance Effect

Magnetic tunnel junctions (MTJs) have attracted strong research interest within the last decades due to their potential use as nonvolatile memory such as magnetoresistive random access memory as well as for magnetic logic applications. Half-metallic magnets (HMMs) have been suggested as ideal electrode materials for MTJs to achieve an extremely large tunnel-magnetoresistance (TMR) effect. Despite their high TMR ratios, MTJs based on HMMs do not exhibit current rectification, i.e., a diode effect, which was achieved in a magnetic tunnel junction concept [E. \ifmmode \mbox{\c{S}}\else \c{S}\fi{}a\ifmmode \mbox{\c{s}}\else \c{s}\fi{}\ifmmode \imath \else \i \fi{}o\ifmmode \breve{g}\else \u{g}\fi{}lu et al., ACS Appl. Electron. Mater. 1, 1552--9 (2019)] based on HMMs and type-II spin-gapless semiconductors (SGSs). The proposed concept has recently been experimentally demonstrated using Heusler compounds. In the present work, we investigate from first-principles MTJs based on type-II SGS and HMM quaternary Heusler compounds $\mathrm{Fe}\mathrm{V}\mathrm{Ta}\mathrm{Al}$, $\mathrm{Fe}\mathrm{V}\mathrm{Ti}\mathrm{Si}$, $\mathrm{Mn}\mathrm{V}\mathrm{Ti}\mathrm{Al}$, and $\mathrm{Co}\mathrm{V}\mathrm{Ti}\mathrm{Sb}$. Our ab initio quantum transport calculations based on a nonequilibrium Green's function method have demonstrated that the MTJs under consideration exhibit current rectification with relatively high on:off ratios. We show that, in contrast to conventional semiconductor diodes, the rectification bias voltage window (or breakdown voltage) of the MTJs is limited by the spin gap of the HMM and SGS Heusler compounds. A unique feature of the present MTJs is that the diode effect can be configured dynamically, i.e., depending on the relative orientation of the magnetization of the electrodes, the MTJ allows the electrical current to pass either in one or the other direction, which leads to an inverse TMR effect. The combination of nonvolatility, reconfigurable diode functionality, tunable rectification voltage window, and high Curie temperature of the electrode materials makes the proposed MTJs very promising for room-temperature spintronic applications and opens ways to magnetic memory and logic concepts as well as logic-in-memory computing.

Quantum transport of short-gate MOSFETs based on monolayer MoSi2N4.

The high carrier mobility, appropriate band gap and good environmental stability of two-dimensional (2D) MoSi2N4 enable it to be an appropriate channel material for transistors with excellent performance. Therefore, we predict the performance of double-gate (DG) metal-oxide-semiconductor field-effect transistors (MOSFETs) based on monolayer (ML) MoSi2N4 by ab initio quantum-transport calculations. The results show that the on-state current of the p-type device is remarkable when the gate length is greater than 4 nm, which can meet the high performance requirements of the International Technology Roadmap for Semiconductors (ITRS), 2013 version. Moreover, the gate length can be reduced to 3 nm when an underlap (UL) structure is employed in the MOSFET, and the sub-threshold swing, intrinsic delay time and power consumption also perform well. The calculation results reveal that ML MoSi2N4 will be a promising alternative for transistor channel materials in the post-silicon era.

The in-plane graphene and boropheneβ12 contacted sub-10 nm monolayer black phosphorous Schottky barrier field-effect transistors

Using ab initio quantum transport calculations, we investigate the performance of the in-plane (IP) graphene and boropheneβ12 contacted sub-10 nm (gate length = 5.1, 6.1, 7.3, 8.8 nm) single-gated (SG) and double-gated (DG) monolayer (ML) black phosphorous (BP) Schottky barrier field-effect transistors (SBFETs). The transfer characteristics, total gate capacitance, intrinsic delay time and dynamic power indicator are studied. The calculated on-state currents of the IP graphene contacted sub-10 nm SG ML BP SBFETs are 1064.8, 1272.3, 1676.5, 1363.8 μA/μm, which are far beyond the requirements of International Technology Roadmap for Semiconductor (ITRS) high performance (HP) application targets. Compared with graphene electrode, the on-state currents of IP boropheneβ12 contacted sub-10 nm SG ML BP SBFETs can only satisfy about 10%–70% requirement of HP standards because of the strong metal induced gap states (MIGS) in the channel. Moreover, we further investigated the effect of DG model on the device performance. The results indicated that the gate electrostatic control is significantly improved by using DG model. The on-state currents of IP graphene and boropheneβ12 contacted sub-10 nm DG ML BP SBFETs are increased by a factor of 1.37–1.56 and 1.16–3.59 compared with SG model. As a result, the large on-state currents of IP configurations based on graphene/ML BP/graphene can greatly stimulate the potential of BP transistors.

Giant magnetoresistance due to orbital-symmetry mismatch in transition metal benzene sandwich molecules

Achieving large magnetoresistance (MR) effects in atomic-scale junctions is of great importance in the field of molecular spintronics. In this work, using ab initio quantum transport calculations, we report on the spin transport properties of molecular junctions made of single transition metal benzene sandwich molecules $(\mathrm{TM}{\mathrm{Bz}}_{2})$ bridged by ferromagnetic electrodes. We find that relative magnetic orientations of two electrodes and the molecule can dramatically affect low-bias transport properties of the junction. While the $\mathrm{TM}{\mathrm{Bz}}_{2}$ are insulating in the gas phase, their corresponding molecular junctions bridged between the two electrodes are half-metallic minority-spin conductors (i.e., almost 100% spin polarization) and display very large MR. More interestingly, even larger molecular MR, corresponding to a reversal of molecular spin moment, are found due to the spin-valve behavior of the molecule controlled by the magnetic impurity. Such a particular performance is attributed to strong spin-selective hybridization of electronic states at the molecule/metal interface due to orbital-symmetry mismatch between the electrode's and molecular states. We also discuss the importance of the shape of electrodes on spin transport efficiency.

Sub-5 nm monolayer black phosphorene tunneling transistors

The successful fabrication of sub-5 nm 2D MoS2 field-effect transistors (FETs) announces the approaching post-silicon era. It is possible for tunneling field-effect transistors (TFETs) based on monolayer black phosphorene (ML BP) to work well in the sub-5 nm region because of its moderate direct band gap, anisotropic electronic properties and high carrier mobility. We simulate the device performance limit of the ML BP TFETs at the sub-5 nm scale using ab initio quantum transport calculations. We predict that the on-state currents (Ion) of the sub-5 nm ML BP TFETs will exceed those of the ML WTe2 TFETs, which possess the highest Ion among the transition-metal dichalcogenide family. In particular, the Ion of the ML BP TFETs can fulfill the 2028 requirements of the international technology roadmap for semiconductors (ITRS) for the high-performance (HP) devices until the gate length is scaled down to 4 nm, while the delay times and power dissipations always surpass the 2028 requirements of the ITRS HP devices significantly in the whole sub-5 nm region.

Sub-10 nm vertical tunneling transistors based on layered black phosphorene homojunction

Van der Waal (vdW) stacking of two-dimensional (2D) semiconductors owns a natural configuration for construction of vertical (interlayer) tunneling field effect transistors (TFETs). We simulate the vertical TFETs composed by layered black phosphorene (BP) homojunction at sub-10 nm scale from the ab initio quantum transport calculations. For high-performance (HP) application, the on-state currents (Ion) of the vertical BP TFETs outperform those of their planar counterparts under similar gate length (Lg) at sub-5 nm scale. Remarkably, the vertical BP TFETs extend the application field to low-power (LP) devices compared with their planar counterparts. Both Ion (LP) and Ion (HP) of the vertical BP TFETs can fulfill the LP and HP requirements of the international technology roadmap for semiconductors (ITRS) until Lg is scaled down to 5 and 3 nm, respectively.

Charge transport in doped zigzag phosphorene nanoribbons

The effects of lattice distortion and chemical disorder on charge transport properties of two-terminal zigzag phosphorene nanoribbons (zPNRs), which shows resonant tunneling behavior under an electrical applied bias, are studied. Our comprehensive study is based on ab initio quantum transport calculations on the basis of the Landauer theory. We use nitrogen and silicon substitutional dopant atoms, and employ different physical quantities such as the $I\text{\ensuremath{-}}V$ curve, voltage drop behavior, transmission spectrum, transmission pathway, and atomic current to explore the transport mechanism of zPNR devices under a bias voltage. The calculated transmission pathways show the transition from a ballistic transport regime to a diffusive and in some particular cases to localized transport regimes. Current flowing via the chemical bonds and hopping are monitored; however, the conductance originates mainly from the charge traveling through the chemical bonds in the vicinity of the zigzag edges. Our results show that in the doped systems, the device conductance decreases and the negative differential resistance characteristic becomes weak or is eliminated. Besides, the conductance in a pure zPNR system is almost independent of the ribbon width.

High performance silicene nanoribbon field effect transistors with current saturation

We investigate field effect transistors (FETs) based on semiconducting armchair-edged silicene nanoribbons (ASiNRs) by using ab initio quantum transport calculations. These FETs have high performance with an I on/I off ratio of over 106 and a subthreshold swing as small as 90 mV/decade. Impressively, the output characteristic shows a saturation behavior. The drain-current saturation is an advantage with respect to device speed, but it’s usually absent in carbon-based (e.g., graphene, graphene nanoribbons, carbon nanotubes, and organic single-molecule) FETs.

Negative rectification and negative differential resistance in nanoscale single-walled carbon nanotube p-n junctions

We investigate the transport properties of few-nm-long single-walled carbon nanotube (SWCNT) p-n junctions for the first time by using ab initio quantum transport calculations. Unlike the previously reported few-μm-long SWCNT p-n junctions, which rectify positively, all the investigated ultrashort SWCNT p-n junctions show negative rectification effect, accompanied by negative differential resistance.

Sign-changeable spin-filter efficiency and giant magnetoresistance in seamless graphene nanoribbon junctions

Sign-changeable spin-filter efficiency is predicted in both the ferromagnetic and antiferromagnetic configurations of a zigzag graphene nanoribbon bridging two half-planar graphene electrodes from ab initio quantum transport calculations. By changing edge spin-polarization configuration, we obtain giant room-temperature magnetoresistance, which is one order of magnitude larger than the maximum experimental results.

Functionalized Metallic Single-Walled Carbon Nanotubes as a High-Performance Single-Molecule Organic Field Effect Transistor: An ab Initio Study

We propose a novel single-molecule organic field effect transistor (FET) fabricated via covalent functionalization of an individual metallic single-walled carbon nanotube (SWCNT). The transfer characteristic of this FET is calculated by using ab initio quantum transport calculations. Because of the significantly reduced screening effect of the quasi-one-dimensional electrode and seamless connection between the electrode and scattering region, the optimized device shows an excellent overall performance over the experimental single-molecule organic field effect transistors. This renders functionalized metallic SWCNTs a promising candidate for a high-performance single-molecule organic field effect transistor.

Electronic structure and transport properties of atomic NiO spinvalves

Ab initio quantum transport calculations show that short NiO chains suspended in Ni nanocontacts present a very strong spin-polarization of the conductance.The generalized gradient approximation we use here predicts a similar polarization of the conductance as the one previously computed with non-local exchange, confirming the robustness of the result. Their use as nanoscopic spinvalves is proposed.